1. Field of the Invention
The invention relates to aluminum alloys containing very finely dispersed metals which have very low solubility in solid aluminum, such as Bi, Cd, In and Pb, and to a process for solidification of such alloys.
2. Description of Related Art
For numerous years, evolution in science and technology has led to the development and marketing of increasingly higher-performance aluminum alloys. The improved performance has been achieved specifically by defining ever-narrower and more precisely targeted ranges of compositions for these alloys, which include and incorporate very small amounts of chemical elements that are also within a very narrow range of composition.
Refined aluminum intended for the manufacture of electrolytic capacitors, whose performance could be improved considerably by incorporating traces (fractions of ppm or ppm) of certain elements such as bismuth, cadmium, indium and lead, may be cited as an extreme example of this progress.
Examples of the favorable effect of dopings with traces of these metals are described in numerous documents, particularly JP 53-114059 (SHOWA AL), JP 54-043563 (SHOWA AL), JP 57-057856 (SHOWA AL), JP 57-110646 (SUMITOMO AL and TOYO), JP 63-288008 (SUMITOMO LIGHT METALS) and JP 1-128419 (SUMITOMO LIGHT METALS).
Although these patents define the desirable dopings broadly enough, they do not specify a practical way of achieving them, nor do they specify the preferred ranges of contents, which in practice would be very narrow.
The widely accepted way of carrying out such very small and very precise additions of elements favorable for the final utilization of the metal consists of the addition, fusion and dispersion of master alloys which contain these favorable elements into the liquid aluminum alloy bath to be optimized, in such quantities that the final content of favorable elements in the molten metal is within a range that is considered optimal.
However, applicants have ascertained that this widely accepted method of operation using master alloys available in the trade, even those which are very pure, led to erratic and extremely variable results which were not compatible with an optimization of the final properties required of a product produced in this way, particularly in the case of the addition of metals in the group bismuth, cadmium, indium and lead to aluminum in quantities which do not exceed 10 parts per million in the final alloy.
By examining the factors which could explain such an excessive variability of results, applicants have ascertained that its chief origin could be an insufficient homogeneity of composition of the master alloys used.
Generally, these commercially available master alloys are obtained by means of natural solidification of the molten master alloy into ingot molds in order to obtain molded pieces which are usable for the desired correction of the composition. These molded pieces most often occur in the form of molded plates with a thickness of several centimeters, which can possibly be fractionated, or cast ingots weighing several hundred grams.
But a careful examination of these products by applicants showed that heavy filler metals such as bismuth, cadmium, indium and lead which have low melting points, are not very soluble in solid aluminum and are very dense, were abnormally distributed in a very heterogeneous way, and were most often present in the form of globules or crystals with sizes larger than 20 micrometers and sometimes larger than 1 mm.
It was reasonable, then, to think that such large-size and very dense globules or crystals could remain trapped by their density at the bottom of a smelting furnace, and that the small specific surface area of large globules or crystals of filler metal thus deposited could result in very low rates of dissolution and diffusion of these dense filler metals in the less dense liquid aluminum alloy bath, thus leading to very erratic and variable final contents of these metals.
The problem to be solved, then, was to produce master alloys containing bismuth, cadmium, indium and/or lead in which these dense and not very aluminum-soluble elements would be very finely dispersed in the aluminum matrix, in a very homogeneous manner throughout the total volume.
If the phase diagrams of the binary alloys Al-Bi, Al-Cd, Al-In, and Al-Pb shown, respectively, in FIGS. 1a, 1b, 1c, and 1d are examined, it can be ascertained that these diagrams are highly similar, and that consequently Bi, Cd, In and Pb form a very specific and very homogeneous group of aluminum alloy elements.
The essential point which would largely explain the practical difficulties encountered is that the alloys of aluminum with these metals which are not very soluble in the solid state exhibit a separation phenomenon in the liquid state (the zones designated L1+L2 in the phase diagrams), implying that the usual master alloys of aluminum with these metals are inevitably diphasic and heterogenous in the solidified state, and include zones which are very rich in alloying metals, and thus very poor in aluminum. Aluminum which is poor in alloying metals would solidify first, "rejecting" a liquid which is very rich in dense alloying metals, this rich liquid having a tendency to collect in large heterogenous globules as a result of the forces of surface tension and gravity.
It therefore appeared unreasonable to attempt to obtain master alloys which included non-negligible contents of additions of "heavy" metals belonging to the group Bi, Cd, In, and/or Pb, in which these metals would be very finely dispersed in the aluminum matrix. A survey of the products available on the market confirmed this analysis.